A combustion engine uses a piston to compress an air-fuel mixture in the cylinder and to subsequently ignite said air-fuel mixture. The increase in volume caused by the explosion of the air-fuel mixture drives the piston, which provides the mechanical energy (power stroke) that can be used in different ways. The efficient operation of a combustion engine that is not self-igniting depends greatly on the air-fuel mixture being ignited at the correct point in time. Ignition plugs are used for igniting the air-fuel mixture and said ignition plugs protrude in part into the cylinder and ignite the air-fuel mixture by means of a spark that is generated across a spark gap that is defined by means of two or more electrodes. A middle electrode in the ignition plug is influenced by a high voltage that is dissipated by way of the ground electrode.
In order to ensure that the spark is actually formed across the spark gap, the rod-shaped middle electrode is surrounded in a radial manner by the insulator that is embodied from a ceramic material or comprises a ceramic material. The rod-shaped middle electrode is only free in the axial direction. In this way, the spark may be prevented from bypassing the ground electrode and jumping directly from the middle electrode to the housing that is typically manufactured from metal.
Known ignition plugs are illustrated by way of example in U.S. Pat. No. 2,691,971 A, WO 2010/043543 A1, US 2013/0229102 A1 and DE 10 2014 103 308. As a result of the high voltage at which the middle electrode is operating, the middle electrode heats up and this causes said middle electrode to expand as a result of its comparatively high thermal coefficient of expansion. The gap prevents the insulator from being damaged as a result of this expansion. In order to ensure that the expansion does not damage the insulator, the gap is produced with a defined over-dimensioning so that even when the middle electrode is in the heated state, it does not come into contact with the insulator. Nonetheless, the gap is so small that oil cannot seep into the gap. Furthermore, the gap ensures that the middle electrode that becomes very hot during the operation is thermally separated from the insulator since air has a comparatively low thermal conductivity characteristic.
With a view to attempting to reduce fuel consumption and consequently the CO2 emissions, modern combustion engines are provided with a cylinder deactivation facility so that one or more switchable cylinders of the combustion engine can be deactivated depending upon the power output requirement and thus operated in the deactivated mode in which the cylinder does not contribute to the power output. The term activated mode describes the mode in which the switchable cylinders contribute to the power output. Since the ignition plug is also switched off in the deactivated state, the middle electrode cools down which results in the gap increasing in size and consequently oil can now pass into the gap. A larger quantity of air-fuel mixture may collect in the gap, said mixture comprising components of oil. When the cylinder is reactivated, the middle electrode heats up almost suddenly and the gap becomes smaller. The oil that is in contact with the middle electrode becomes coked as a result of the middle electrode heating up rapidly and as a result solid deposits are formed in the gap. The deposits that increase in size over time can damage the insulator. As mentioned before, the insulator frequently comprises a ceramic material that is brittle so that the insulator can tear as the cylinder is activated and pieces can break off from the insulator. In the worst case scenario, the middle electrode is no longer insulated with respect to the wall of the housing such that the high voltage may bypass the ground electrode and discharge directly from the middle electrode to the housing, which causes undesired misfires.
The inventors herein have recognized the above issues and identified an approach by hhich the issues described above may be at least partly addressed. It is therefore the object of the present disclosure to develop the ignition plugs such that the insulator is protected if the ignition plug is used in the combustion engines that have a cylinder deactivation facility.
In accordance with the disclosure, the ignition plug comprises a sealing element that can be activated based on the mode of the cylinder so as to seal the gap. As already mentioned, the gap in the activated mode is so small that oil cannot seep into the gap. If a switchable cylinder is in the deactivated state, then the sealing element is activated and the gap that is then increasing in size is sealed, as a result of which oil is prevented from passing into the gap. It is not necessary for the gap to be completely sealed. On the contrary, it is sufficient that the gap is reduced in size to the extent that oil can no longer pass into the gap. This optimal gap may be termed as gap seal. Consequently, deposits that could damage the insulator are prevented from forming. It is also possible in accordance with the disclosure to operate the ignition plug over a longer period of time at full functional capability in combustion engines that comprise a cylinder deactivation facility.
The sealing element can be activated by an ignition coil, which cooperates with the ignition plug, or by an open-loop and closed-loop unit for controlling the combustion engine in an open-loop and/or closed loop manner. The high voltage is typically provided by an ignition coil that is connected in a conductive manner to the ignition plug. As described above, the ignition plug is likewise not operated when the cylinder is in the deactivated mode so that the idle operating state of the ignition coil can be used as a signal to activate the sealing element. Modern combustion engines comprise a complex electronic open-loop and closed-loop control unit that may control the ignition sequence of the cylinders by means of correspondingly controlling the ignition coils that needs to be correspondingly changed as one or more cylinders are switched off. The open-loop control unit can consequently also be used for activating or deactivating the sealing element. In this respect, components that are already provided are used to activate or deactivate the sealing element so that the constructive additional outlay can be kept to a minimum.
In one example, the sealing element may comprise a heating device for heating the middle electrode. As mentioned before, the gap between the middle electrode and the insulator increases in size as a result of the middle electrode cooling down after a switchable cylinder has been deactivated. Since the middle electrode must be embodied from an electrically conductive material, it is usually embodied from a metal and for this reason the middle electrode has a relatively high thermal coefficient of expansion. In the case of a cylinder that has been deactivated, the middle electrode can be heated with the aid of the heating device so that the size of the gap that is formed in the deactivated mode can be kept small enough such that oil cannot pass into the gap. In this respect, the middle electrode acts as a sealing body that can be used to seal the gap. Thereby, the above described disadvantageous effects of the deposits that form in the gap are eliminated or are considerably reduced in comparison to an ignition plug that does not comprise a heating device. In this way, the ignition plug can be operated over a longer period of time at full functional capability.
In one example embodiment, the heating device can comprise heating wires that run in the middle electrode. Heating wires comprise a high electrical resistance so that they heat up under the influence of an electrical current. As mentioned before, ignition coils that provide a high voltage are typically provided. The heating wires can be connected to the ignition coils and are influenced by said ignition coils with a corresponding voltage or a corresponding current. When the heating wires are not directly connected to the ignition coils, the electronic system for the ignition coil can at least be used so that additional installation space is not required. Furthermore, the heating wires can be connected by way of example by connecting pins to existing cable harnesses if said cable harnesses are providing a sufficiently high current. As a consequence, the middle electrode can be heated up without the need for additional components. The additional outlay for converting the solution in accordance with the disclosure is consequently kept to a minimum which applies in particular to the outlay with regard to the cabling since existing lines and cable harnesses can be used.
The object is further achieved by means of a method for operating an ignition plug so as to initiate combustion in a switchable cylinder of a combustion engine, wherein the switchable cylinder can be operated in an active mode and in a deactivated mode. The ignition plug comprises a housing that can be connected to the cylinder, an insulator that is fastened to the housing and comprises an axial bore hole, a middle electrode that is arranged in the axial bore hole to form an annular gap between the middle electrode and the insulator, a ground electrode that is fastened to the housing, and a sealing element that can be activated so as to seal the gap. The method comprises activating the sealing element when the cylinder is being operated in the deactivated mode and deactivating the sealing element when the cylinder is being operated in the activated mode.
In this way by virtue of the possibility of sealing the gap at least in the deactivated mode of the cylinder, the collection of oil and formation of deposits in the gap are prevented or at least can be considerably reduced so that the damage to the insulator is avoided when a switchable cylinder is returned to the activated mode. In comparison to conventional ignition plugs for combustion engines that have a cylinder deactivation facility, the ignition plug can be operated over a longer period of time at full functional capability.
As described above, when the cylinder is in the deactivated mode, oil can pass into the gap causing deposits to form on the middle electrode. Since this effect does not occur when the switchable cylinder is in the activated mode, the sealing element itself can be deactivated when the cylinder is in the activated mode, as a result of which energy can be saved depending upon the design of the sealing element.